Analysis and prevention measures for cracks that occur when the excavator sprocket is quenched using a medium-frequency induction heating power supply

The sprocket is the main transmission component of the tile assembly, and its material is SCSIMN2H steel. The sprocket receives great bending stress and impact stress during work. At the same time, it comes into contact with sand, gravel and soil during excavation, and is subject to strong wear and tear. Therefore, the sprocket is required to have high impact toughness, high hardness and hardened layer depth. The surface hardness is 40-47HRC, the tooth top hardened layer depth is 18mm, and the tooth root hardened layer depth is ≥3mm. During production, it was found that after the sprocket was quenched with a medium-frequency induction heating power supply, quenching cracks and fractures occurred in batch workpieces.

The heat treatment of the sprocket adopts the overall medium frequency induction heating power supply spray buried quenching cooling method, and the quenching liquid adopts 4%-6% AQ251 quenching mechanism. The inspection found that cracks appeared at the root of the workpiece. The cracks were distributed longitudinally along the tooth root and were longitudinal cracks. Macroscopic inspection found that there were obvious pores, blisters and other casting defects in the cracked workpiece. These defects caused a serious decrease in the strength and toughness of the workpiece. On the other hand, defects in the sprocket caused the workpiece to change the distribution of magnetic field lines during induction heating, forming a package. The high-density eddy current area in the defective part of the network. The high-density magnetic field lines in the I-6 zone cause heating at the defect boundary of the workpiece, forming a coarse overheated structure after quenching, which reduces the fracture toughness of the workpiece and accelerates the initiation and expansion of cracks in the workpiece under external force. The inspection found that the normalizing and holding time of the sprocket was insufficient, so the workpiece structure failed to be refined, and the coarse grains that existed in the process were retained. After quenching, a coarse martensite structure was produced. This structure will collapse under small tensile stress. Quenching cracks and cracking occur. The direct cause of cracks in the sprocket is the tensile stress caused by the different heating depth and hardening depth of the tooth top and tooth root. The tooth root heating layer is shallow, while the tooth top heating layer is quite deep, about 6 times the depth of the tooth root heating layer. During cooling, the structural transformation of the tooth root is completed quickly, forming a martensite structure, which is in a state of residual compressive stress, while the tooth top is still undergoing cooling transformation. During the continued cooling, the tooth top and tooth root are mainly affected by thermal stress, and the tooth top forms a tensile stress state on the tooth root during contraction. When the residual tensile stress is greater than the fracture strength of the tooth root of the workpiece, quenching cracks and cracking failure will occur at the tooth root.

After comprehensive analysis, the measures to prevent quenching cracks in the sprocket medium frequency induction heating power supply are proposed as follows:

1. Strictly control and inspect the quality of castings to prevent blanks with blisters, pores and other defects from flowing into the product processing process.

2. Strictly control and inspect the sprocket normalizing process and quality.

3. Strengthen the grain size inspection of incoming blanks, and the sampling area is the sprocket tooth root to ensure good product quality.

4. During the medium frequency induction heating and quenching cooling of the sprocket, controlling the appropriate cooling time can effectively reduce the residual tensile stress of product cracks and prevent quenching cracks in the workpiece.

After the sprocket adopts the above preventive measures, the quality of the workpiece is excellent, no quenching crack defects are found, and the production runs normally.